1. Field of the Invention
The present invention relates to a semiconductor arrester which is a safety device for protecting a communication instrument or the like from an abnormally high voltage caused by lightning, contact with a power cable, or short-circuit or failure in the communication instrument. Particularly, it relates to a semiconductor arrester of diode-bridge thyristor type.
2. Description of the Related Art
FIG. 9 shows the circuitry of a semiconductor arrester of diode-bridge thyristor type (hereinafter referred to as "a semiconductor arrester"), having three terminals (T; tip, E; earth, R; ring), six diodes (D1 to D6) and one reverse-blocking diode thyristor (T1), wherein an anode of D1 and a cathode of D2 are connected with terminal T (or terminal R); an anode of D6 and a cathode of D5 are connected with terminal E; cathodes of D1, D3 and D6 are connected with an anode of T1 and anodes of D2, D4 and D5 are connected with a cathode of T1.
In such a circuit structure, if it is assumed that an abnormally high voltage of positive polarity is applied, for example, to terminal T, then D1, T1 and D5 are sequentially biased in the forward direction and T1 is turned on to connect terminal T to terminal E via a low resistance. Accordingly, the abnormally high voltage can be released to terminal E (earth) so that the communication instrument connected to terminal T is protected.
FIG. 10 shows a package structure for the conventional semiconductor arrester, seen from above when a cap is removed, wherein the first to the sixth metallic members 11 through 16 are mounted on an insulating base plate 10.
The first, second, fifth and the sixth metallic members 11, 12, 15 and 16 all penetrate the insulation base plate 10 from the front side to the back side thereof. The first metallic member 11 exposed on the back side of the insulation base plate 10 is used as terminal T (or terminal R), and the second metallic member 12 on the back side is as terminal R (or terminal T). Also, the fifth metallic member 15 and the sixth metallic member 16 on the back side are both used as terminal E or one of them is as terminal E while being connected with the other.
The first metallic member 11 on the front side of the insulation base plate 10 is used as anode contact of D1 and cathode contact of D2; the second metallic member 12 on the front side is as anode contact of D3 and cathode contact of D4; the fifth metallic member 15 on the front side is as cathode contact of D5; and the sixth metallic member 16 on the front side is as anode contact of D6.
Further, the third metallic member 13 is used as anode contacts of D2, D4 and D5 as well as cathode contact of T1; and the fourth metallic member 14 is as cathode contacts of D1, D3 and D6 as well as anode contact of T1.
In the conventional package structure of semiconductor arrester described above, since the elements were actually arranged in the circuitry as they are illustrated in FIG. 9, there is a problem in that, for example, D5, T1 and D6 are located in one row to require a longer space, and the thyristor and the diodes are not base chips, but are thickly covered, whereby they can be installed only in a relatively large package.